1. Field of the Invention
The invention relates to an apparatus for degassing a liquid or pasty medium in a machine for producing and/or upgrading a material web, in particular a web made of paper or cardboard.
2. Description of the Related Art
Such a degassing apparatus typically includes a vessel rotable about an axis of rotation, a medium-supply device for introducing the medium to be degassed into the vessel, a medium-discharge device for discharging a lower-gas fraction of the medium out of the vessel, and a gas-discharge device for discharging a gas-rich fraction of the medium out of the vessel.
An apparatus of this type is known, for example, from the dissertation by Dipl.-Ing. Guinter Schadel entitled, xe2x80x9cEine Zentrifuge zum Abscheiden kleiner Gasblasen aus hochviskosen Flxc3xcissigkeitenxe2x80x9d [xe2x80x9cA centrifuge for separating small gas bubbles from high-viscosity liquidsxe2x80x9d] (Faculty of Chemical Engineering of the Fridericana University Karlsruhe, 1979; University Library Karlsruhe 79 DA 30 C). In this apparatus, which operates in the manner of a centrifuge, the medium to be degassed is supplied to the inner wall of a rotating vessel. In this case, however, the medium covers the inner wall only as a thin film. The result of this is that, for a predetermined degassing quality and a predetermined quantity of medium to be degassed per unit time, the known apparatus requires a disproportionately large amount of construction space. The apparatus proposed by Schxc3xa4del has therefore not gained acceptance in practice.
Instead, cyclones have been used conventionally (see, for example, EP 0 618 012 A1 and U.S. Pat. No. 5,080,792) in the prior art for the degassing of liquid or pasty media. In cyclones of this type, the medium to be degassed is introduced eccentrically into a stationary vessel, so that a swirling movement of the medium is established in the interior of the vessel. The centrifugal force caused by this swirling movement ensures segregation of the medium and the gas by utilizing the density differences between the liquid or pasty medium and the gas included in it. A disadvantage of this, however, is that the force bringing about this segregation has to be generated by the kinetic energy of the inflowing medium, thus leading to a high pressure drop or pressure loss at the cyclone. In order to compensate for this pressure loss, therefore, correspondingly high-performance pumps have to be provided in order to convey the medium to be degassed.
The present invention therefore, relates to a degassing apparatus of the generic type, which, while requiring a small amount of construction space, is capable of degassing per unit time even large quantities of liquid or pasty medium to a degree sufficient for practice that yet gives rise, at most, to a pressure drop which does not exceed an acceptable amount.
The present invention sets forth, in one embodiment, a degassing apparatus in which the gas-discharge device includes an immersion-tube arrangement having one or more immersion tubes, the inlet orifice of each being arranged so as to be at least adjacent to the axis of rotation of the vessel. The use of such an immersion-tube arrangement for extracting the gas-rich fraction out of the rotating vessel makes it possible to discharge this gas-rich fraction in a controlled manner, irrespective of the operating conditions (rotational speed) and of the design parameters (wall height and wall inclination) of the rotating vessel.
The quantity of gas-rich fraction discharged per unit time can be influenced by the choice of the immersion-tube diameter and, if appropriate, by varying the suction capacity of a pump which can be connected to the immersion-tube arrangement. Furthermore, by use of the immersion-tube arrangement, a vacuum can be applied locally in order to assist in the segregation of the medium. Thus, the degassing quality can be improved by setting the operating parameters of the vessel and the extraction parameters on the immersion-tube arrangement in adaptation to the respective medium. It may be advantageous, at the same time, to arrange the immersion-tube arrangement at least in the vicinity of the axis of rotation of the vessel, so that an agitating action and an accompanying renewed intermixing of the gas-rich and low-gas fractions can be avoided.
In a development of the invention, the inlet orifice of the immersion-tube arrangement may be assigned a grid structure which extends preferably over a predetermined distance in the direction of the discharge of the gas-rich fraction. This affords the possibility of breaking the envelope of gas bubbles contained in the gas-rich fraction as early as when the latter passes through the grid structure.
Consequently, the separation of the medium and the gas is achieved more easily and/or more quickly.
So that the apparatus can be operated with as little loss of medium as possible, the gas-discharge device may lead the gas-rich fraction of the medium to a collecting vessel, out of which, in turn, the medium-supply device is fed.
An inlet orifice of the medium-discharge device may extend essentially over the entire circumference of the vessel. This is advantageous because a larger quantity of low-gas medium can thereby be discharged per unit time, as compared with a locally concentrated medium discharge.
Moreover, the medium-discharge device may have a plurality of guide vanes which are rotatable jointly with the vessel. By use of these guide vanes, the flow of the low-gas fraction leaving the vessel through the outlet orifice is influenced. The guide vanes bring about a reduction in the degree of turbulence in the flowing medium upon which they act. Preferably, after passing the guide vanes, the medium is in a laminar flow state. The tendency of the medium to form flow-induced bubbles can thereby be reduced by passing it over the guide vanes, which has a positive effect on the degassing result.
The rotating vessel may be designed in such a way that it widens, preferably conically, from an outlet orifice of the medium-supply device into the vessel toward an inlet orifice of the medium-discharge device out of the vessel. Since the centrifugal forces acting on the medium inside the rotating vessel increase linearly in the radial direction, the segregation action, particularly in the region of the outlet of the low-gas fraction out of the vessel, can be reinforced by incorporating such a widening of the vessel.
So that gravity-induced asymmetries of the forces inside the vessel which act on the medium can be avoided, the vessel may advantageously be arranged in such a way that its axis of rotation runs essentially vertically during operation. Furthermore, so that undesirable loads (i.e., unbalances) on the apparatus due to its operation can be reduced, it is advantageous for the axis of rotation of the vessel to run through the interior of the latter. An even greater reduction in the above-mentioned loads can be achieved by designing the vessel to be essentially rotationally symmetrical and the axis of rotational symmetry thereof to coincide essentially with its axis of rotation.
For a further improvement in the result of the degassing of the medium, the outlet orifice of the medium-supply device may be arranged at the lower end of the vessel when the latter is in operation, while the inlet orifice of the medium-discharge device may be arranged at the upper end thereof As a result, the dwell time of the medium in the vessel and, consequently, the degassing quality can also be increased, since, with this arrangement, the medium is delayed by gravity on its way from the medium-supply device to the medium-discharge device of the vessel.
A particularly good degassing result can be achieved with vessels which have a diameter of approximately 40 cm and are operated at a rotational speed of about 300-3000 revolutions per minute and preferably at a rotational speed of about 2000 revolutions per minute. While the degassing apparatus is in operation, the degree of filling of the vessel may amount to about 20%, so that effective operation of the plant, above all as regards the gas-discharge device, can be ensured.
According to a second embodiment, the invention includes a method for degassing a liquid or pasty medium in a machine for producing and/or upgrading a material web, in particular a web made of paper or cardboard. Specifically, the medium to be degassed is introduced into a vessel rotating about an axis of rotation, a low-gas fraction of the medium is discharged from the vessel and a gas-rich fraction of the medium is discharged from the vessel separately from said low-gas fraction, the vessel being operated with a degree of filling of up to about 20%.
Schxc3xa4del expressly points out, in his dissertation, that, when a centrifuge is used, the medium to be degassed should be exposed to the centrifugal force field only as a thin film on the inner wall of the centrifuge vessel. The reason for given by Schadel for this is that, with an increase in the layer thickness of the medium, the centrifugally induced hydrostatic pressure prevailing in the medium also rises, particularly in the near-wall regions of the layer. This, allegedly, on the one hand, reduces the rate of ascent of the small gas bubbles and, on the other hand, leads to some of the gas being physically dissolved in the medium again.
It is to the inventor""s credit that he has overcome this prejudice which has existed amongst specialists for 20 years. Assuming the same throughput quantity per unit time, an increase in the degree of filling of the centrifuge vessel leads to an increase in the dwell time of the medium in the vessel due to the buffer effect of the vessel volume. This has a beneficial effect on the degassing quality.